The present invention relates to a circuit for separating a composite video signal into a luminance signal and a chrominance signal in a color television, and more particularly to a correlation adaptive luminance/chrominance signal separating circuit which detects vertical and horizontal correlation with respect to an arbitrary reference composite video signal in a received digital composite video signal and adaptively separates the luminance signal and the chrominance signal according to the correlations.
It is generally known that the human eye distinguish chrominance data such as hue, saturation, etc., with great detail, unlike luminance data. Thus, most television video signals assign luminance data a wide band (4.2 MHz in the NTSC type) as shown in FIG. 1A. In contrast, the resolution for hue and saturation only requires 1/10 to 1/3 bandwidth of the brightness.
According to the aforementioned characteristic, the chrominance signals have a much narrower bandwidth than the luminance signal. For instance, as shown in FIG. 1B, an I signal component of the chrominance signal has a band of 1.3 MHz, and a Q signal component has a band of 0.5 MHz, respectively. Also, in the NTSC type, the chrominance subcarrier has the frequency of 3.579545 MHz, i.e., 227.5 f.sub.H and the sideband of the chrominance signal is interleaved with the high frequency sideband of the luminance signal as shown in FIG. 1C.
When the composite video signal having the spectrum such as FIG. 1C is displayed, dots which continuously exist in an arbitrary scan line are interleaved to the dots of the next line. Thus, even in a black-and-white television receiver, the picture looks normal in the proper watching distance. This is why the NTSC type is compatibly used with the black-and-white television broadcasting type. And in the NTSC color television receiver, a luminance channel is formed by removing a chrominance signal from a composite video signal in which the luminance signal and the chrominance signal are interleaved as shown in FIG. 1D, and a chrominance channel is formed by demodulating the chrominance signal to produce a chrominance difference signal, so that the color television image can be watched.
In a conventional luminance/chrominance signal separating method adapted to a color television receiver, a band reject filter in which a luminance signal is attenuated by about 3 dB between 2.3 MHz and 2.8 MHz and is completely attenuated at 3.58 MHz, as shown in FIG. 1E, and a band pass filter (hereinafter, referred to as BPF) having the center of 3.58 MHz, as shown in FIG. 1F are installed. However, as the result, the resolution of the luminance signal is deteriorated. For instance, the transition of the luminance signal having a rising time of 150 ns is increased to 250 ns through 300 ns. Also, the high frequency component of the luminance signal is wrongly filtered together with chrominance signal in the BPF such as FIG. 1F, thereby generating a cross color phenomenon during the demodulation of the chrominance signal.
Accordingly, the attenuation of the luminance signal is generated in the luminance channel separated from the composite video signal where the luminance signal and the chrominance signal are interleaved, and a hindrance phenomenon is generated in the chrominance channel by the high frequency component of the luminance signal.
To prevent the generation of the factors causing the deterioration of the image and separate the luminance signal and the chrominance signal, a comb-filtering method has been used.
The block diagram shown in FIG. 2 represents a general comb-filter which comprises a 1H (a period of horizontal synchronizing signal; hereinafter, referred to as H) delay circuit 10, subtracters 20 and 50, a 1/2 amplifier 30 and a BPF 40. In the comb-filter, since chrominance signal components existing in two continuous horizontal scanning lines have the phase difference of 180.degree. with respect to each other, the difference between two continuous horizontal scanning lines is obtained and then the luminance component is removed to obtain only the chrominance component. However, in the case of a vertical transition of the image, the luminance signal of low band (0 to approximately 2.3 MHz) is filtered with the chrominance signal, thereby reducing the vertical resolution of the luminance signal and also generating a cross color phenomenon during the demodulation of the chrominance signal. Thus, the comb-filter band-pass-filters the signal obtained from the difference between two horizontal scanning lines to separate the chrominance signal and removes this chrominance signal from the composite video signal to obtain a luminance signal.
In the comb-filter of FIG. 2, when a composite video signal V is input, it is first delayed by 1H in the 1H delay circuit 10. And the difference between the 1H delayed video signal V.sub.H and the currently input video signal V is obtained in the first subtracter 20, so that the luminance signals of the two signals are counterbalanced to each other the chrominance signals are added and then output. The 1/2 amplifier 30 1/2-amplifies the signal outputted from the first subtracter 20 to obtain the average level of the chrominance signal. Then, the signal output from the 1/2 amplifier 30 is band-pass-filtered in the BPF 40 to obtain only the chrominance signal, and the second subtracter 50 subtracts the chrominance signal outputted from the BPF 40 from the currently input video signal V, so that the luminance signal is obtained.
However, in the comb-filter, a hanging dot phenomenon is generated by the vertical transition of the chrominance signal and the dot crawl phenomenon in the luminance channel is generated by the chrominance signal mixed in the luminance signal Y in the band pass filtering step. Accordingly, several methods are suggested to prevent the deterioration of an image and a well-known method among them is a correlation adaptive comb-filtering method.
FIG. 3 illustrates one of the conventional correlation adaptive luminance/chrominance signal separation circuits which includes first and second 1H delay circuits 60 and 70, subtracters 80 and 90, first and second 1/2 amplifiers 100 and 110, first and second LPFs 120 and 130, a comparator 140 and a data selector 150. The circuit of FIG. 3 detects each vertical correlation between adjacent horizontal scanning lines among three continuous horizontal scanning lines to comb-filter the composite video signal having the larger correlation. In the circuit of FIG. 3, the currently input video signal V is delayed by 1H in the first 1H delay circuit 60 and the 1H delayed video signal V.sub.H is again delayed by 1H in the second 1H delay circuit 70.
And the third subtracter 80 subtracts the 1H delayed video signal V.sub.H from the currently input video signal V and the fourth subtracter 90 subtracts the 2H delayed video signal V.sub.H H from the 1 H delayed video signal V.sub.H. The signals output from the third and fourth subtracters are respectively 1/2-amplified in the first and second 1/2 amplifiers 100 and 110 to be supplied to the input terminals of the data selector 150. That is, two chrominance signals obtained by respectively comb-filtering the currently input video signal V and the 2H delayed video signal V.sub.H H with respect to the 1H delayed video signal V.sub.H among three continuous horizontal scanning lines are respectively supplied to input terminals of the data selector 150. Also, the first and second LPFs 120 and 130 low-pass-filter signals output from the third and fourth subtracters 80 and 90 to remove the chrominance signal component and supply the signals to input terminals of the comparator 140, respectively. The comparator 140 compares the magnitude of the data input in the two input terminals A and B, and if the data inputted in the terminal A is larger than the data input in the terminals B, the magnitude comparator outputs the logic "1", and if not, it outputs the logic "0".
Also, when there is no vertical transition between adjacent horizontal scanning lines in three continuous horizontal scanning lines, "0" is output from the first and second LPFs 120 and 130, and when there is a vertical transition, the luminance components which are not counterbalanced each other are output from the third and fourth subtracters 80 and 90. Then, when the output signal of the first LPF 120 is greater than the output signal of the second LPF 130, the comparator 140 outputs the logic "0", and accordingly, the data selector 150 selects the signal output from the second 1/2 amplifier 110 and outputs it through the output terminal Z. Then, the chrominance signal output from the data selector 150 is band-pass-filtered by the BPF as shown in FIG. 1 to obtain the separated chrominance signal and the luminance signal is separated using this chrominance signal. As a result, the two horizontal scanning lines having the relatively large vertical correlation are comb-filtered, thereby preventing the hanging dot phenomenon. The circuit of FIG. 3 can prevent the hanging dot phenomenon, but still generates the dot crawl phenomenon.